This invention relates generally to optical disc manufacturing and more particularly, to the manufacture of multilayer optical discs.
Multilayer discs are now being produced that provide two or more layers of grooves or pits representing recorded or stored data or information. Such discs are generally manufactured by first producing two different discs, each having its own layer of recorded information on one side or surface of the disc. The two discs are then joined by applying an adhesive to one or both of the surfaces of the layers of recorded information and then bringing the discs together. The result is a disc having two adjacent layers of the different information in the middle of the disc in which the layers of information are separated by a transparent, intermediate bonding layer. Each of the two different layers is independently read by focusing a reading head of a laser on a layer of data to be read. Such multilayer discs are often used in DVD applications.
As can be appreciated, any air bubbles or other distortion in the intermediate layer of bonding fluid or adhesive can interfere with the reading of the layer of data or information behind the bonding layer, thereby potentially resulting in reading errors. Therefore, it is critical that the bonding layer be of a constant thickness and free of any air bubbles. Air bubbles can be formed in the process of storing, transferring or applying the bonding fluid or adhesive to the discs or, in the process of bringing the discs together after the adhesive has been applied. Several processes are known that attempt to eliminate air bubbles in the disc bonding process. For example, it is known to apply the bonding resin very slowly, so that air bubbles are not formed in the adhesive application process. In another process, the discs are spun at a high speed to remove air bubbles after the adhesive has been applied. In a further process, the discs are oriented and maintained in parallel as they are brought together in order to minimize the opportunity for air bubbles to form in the joining process. In still other processes, after the bonding adhesive is applied, the discs are moved to a vacuum chamber and brought together; and then, the discs are subjected to a high pressure environment in an autoclave.
While all of the above processes for removing air bubbles are successful, thereby improving the quality of the bonded disc, such processes achieve that success at the price of substantially increasing the processing time required to effect the bonding of the discs, or at the price of requiring additional expensive equipment, or both. Consequently, there is still a need for an improved process for joining optical discs with an intermediate layer of bonding fluid that reliably inhibits the formation of air bubbles and substantially reduces the time to implement the bonding process.